A current cellular network has been evolved from a legacy macro cell to a heterogeneous small cell. It is expected that a base station having small coverage, such as a small cell, will be evolved toward an ultra-dense network (UDN) of which coverage becomes small to reach several tens of meters or several meters Small base stations that configure such a UDN may be a set of base stations called a femto cell (Fcell), a pico cell (Pcell) or a small cell.
FIG. 1 illustrates a UDN comprised of a macro cell and a plurality of small access points (APs). In a structure of the UDN illustrated in FIG. 1, one macro cell may serve all user equipments. Also, the macro cell serves to prevent a coverage hole from being generated in a cellular network. For example, the UDN becomes an overlaid network where a plurality of small APs are overlapped to configure coverage and at the same time a macro cell, which manages all the APs, exists to overlap the small APs. This is to prevent service drop such as call drop from being generated, wherein the call drop is caused as a user moves to a coverage hole in the middle of receiving a service, in which real time feature is important, such as voice call.
The macro cell may be used to process control information of all networks. On the other hand, the UDN may mainly be used for capacity boost, and its coverage may be overlapped between small APs. Also, since a considerably large of small APs may be provided, a plurality of small APs having no connected user equipment may exist. The small APs having no connected user equipment may dynamically be powered off depending on a space-time use pattern, whereby energy consumption at the base station can be reduced.